Atomic StructureAQA GCSE Physics (8463)

Atoms and Isotopes

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

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Master the topic

Worked examples5 Definitions11 Common mistakes7

Step-by-step worked examples

01.Comparing nucleus and atom size
Foundation
Question
An atom has a radius of about 10⁻¹⁰ m. Its nucleus has a radius of about 10⁻¹⁵ m. By what factor is the nucleus smaller than the whole atom?
Solution
1. 1
  Divide.
  $10^{-10} / 10^{-15} = 10^5$
2. 2
  Express.
  $\text{Atom is about 100,000 times the diameter of its nucleus (about 10,000× by radius depending on definition).}$
Answer
About 10⁵ (100,000) times larger by diameter — atoms are mostly empty space.
02.Read nuclear notation
Foundation
Question
An atom is written as ²³₁₁Na. State the number of (a) protons, (b) neutrons, (c) electrons in this neutral atom.
Solution
1. 1
  Z = 11.
  $\text{Protons = } Z = 11$
2. 2
  Neutrons.
  $\text{Neutrons = } A - Z = 23 - 11 = 12$
3. 3
  Electrons.
  $\text{Electrons = } Z = 11 \text{ (neutral atom)}$
Answer
11 protons, 12 neutrons, 11 electrons.
03.Compare two isotopes
Higher
Question
Carbon-12 and Carbon-14 are both isotopes of carbon. State what is the same and what is different about their nuclei.
Solution
1. 1
  Same.
  $\text{Same number of protons (6 in each).}$
2. 2
  Different.
  $\text{Different number of neutrons: 6 in C-12, 8 in C-14.}$
3. 3
  Implications.
  $\text{Same chemistry, different mass, different nuclear stability (C-14 is radioactive).}$
Answer
Both have 6 protons. C-12 has 6 neutrons; C-14 has 8 neutrons. Same chemistry but different mass; C-14 is radioactive.
04.Ion formation
Foundation
Question
A neutral oxygen atom (Z = 8, A = 16) gains 2 electrons. Write its symbol with charge and state the number of protons, neutrons and electrons.
Solution
1. 1
  Charge from gained electrons.
  $\text{Gained 2 e}^- \Rightarrow \text{charge} = -2;\quad \text{Symbol: O}^{2-}$
2. 2
  Subatomic counts.
  $\text{Protons} = 8;\quad \text{Neutrons} = 16 - 8 = 8;\quad \text{Electrons} = 8 + 2 = 10$
Answer
O²⁻: 8 protons, 8 neutrons, 10 electrons.
05.Interpret alpha scattering
Higher
Question
Explain how the observations from the Rutherford alpha-scattering experiment led to the nuclear model of the atom.
Solution
1. 1
  Observation 1.
  $\text{Most α particles passed straight through → atom mostly empty space.}$
2. 2
  Observation 2.
  $\text{Some α deflected at small angles → met a region of positive charge.}$
3. 3
  Observation 3.
  $\text{A few bounced straight back → hit a tiny, dense, positive nucleus.}$
4. 4
  Model implications.
  $\text{This is incompatible with plum pudding. The nuclear model replaced it: nucleus is small, dense and positive; electrons orbit at distance.}$
Answer
Most went through (empty space); small deflections (positive charge); rare bounce-back (small dense nucleus). Together → nuclear model with concentrated positive nucleus and mostly empty space.

Practice with flashcards

Card 1 of 110 known · 0 to review

Key definitions and keywords

NucleusExaminer keyword: The dense central region of an atom containing protons and neutrons.
ProtonExaminer keyword: Positively charged subatomic particle in the nucleus. Relative charge +1, relative mass 1.
NeutronExaminer keyword: Neutral subatomic particle in the nucleus. Relative mass 1, no charge.
ElectronExaminer keyword: Negatively charged particle orbiting the nucleus in specific energy levels. Relative charge −1, relative mass ~1/2000.
Atomic number (Z)Examiner keyword: The number of protons in the nucleus of an atom. Defines the element.
Mass number (A)Examiner keyword: The total number of protons and neutrons in the nucleus.
IsotopeExaminer keyword: An atom of the same element (same Z) with a different number of neutrons (different A).
IonExaminer keyword: A charged atom (or molecule), where the number of electrons differs from the number of protons.
Plum-pudding modelExaminer keyword: Thomson's 1897 model: an atom is a sphere of positive 'dough' with electrons embedded like plums.
Nuclear modelExaminer keyword: Rutherford's model: a small, dense, positively-charged nucleus surrounded by mostly empty space in which electrons orbit.
Bohr modelExaminer keyword: Electrons orbit in fixed energy levels (shells); explains electron stability and emission spectra.

Common mistakes and misconceptions

Mistake
Saying nucleus and atom are the same size.
Why it happens
Pictures aren't to scale.
How to avoid it
Atom diameter 10⁻¹⁰ m vs nucleus 10⁻¹⁵ m — nucleus is 100,000× smaller.
Mistake
Treating electron mass as significant.
Why it happens
Each subatomic particle 'counts'.
How to avoid it
Electron mass is ~1/2000 of a nucleon — negligible at GCSE.
Mistake
Confusing Z and A.
Why it happens
Both are numbers on the symbol.
How to avoid it
Z (small number, bottom-left) = protons; A (larger, top-left) = protons + neutrons.
Mistake
Saying isotopes are different elements.
Why it happens
They have different mass numbers.
How to avoid it
Same Z = same element. They are different ATOMS of the same element.
Mistake
Thinking ions have a different number of protons.
Why it happens
Ions feel 'changed'.
How to avoid it
Only electrons change. Protons stay the same (else it would be a different element).
Mistake
Mixing up scientists and contributions.
Why it happens
Many scientists involved.
How to avoid it
Memorise: Dalton (spheres), Thomson (electron, plum pudding), Rutherford (nucleus), Bohr (energy levels), Chadwick (neutron).
Mistake
Saying 'most particles bounced back'.
Why it happens
Confusing the surprising minority with the bulk result.
How to avoid it
Most passed STRAIGHT THROUGH. A tiny fraction bounced back — the surprise that revealed the nucleus.

Atomic StructureAQA GCSE Physics (8463)

Atoms and Isotopes

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

PreviousNext

Master the topic

Worked examples5 Definitions11 Common mistakes7

Step-by-step worked examples

01.Comparing nucleus and atom size
Foundation
Question
An atom has a radius of about 10⁻¹⁰ m. Its nucleus has a radius of about 10⁻¹⁵ m. By what factor is the nucleus smaller than the whole atom?
Solution
1. 1
  Divide.
  $10^{-10} / 10^{-15} = 10^5$
2. 2
  Express.
  $\text{Atom is about 100,000 times the diameter of its nucleus (about 10,000× by radius depending on definition).}$
Answer
About 10⁵ (100,000) times larger by diameter — atoms are mostly empty space.
02.Read nuclear notation
Foundation
Question
An atom is written as ²³₁₁Na. State the number of (a) protons, (b) neutrons, (c) electrons in this neutral atom.
Solution
1. 1
  Z = 11.
  $\text{Protons = } Z = 11$
2. 2
  Neutrons.
  $\text{Neutrons = } A - Z = 23 - 11 = 12$
3. 3
  Electrons.
  $\text{Electrons = } Z = 11 \text{ (neutral atom)}$
Answer
11 protons, 12 neutrons, 11 electrons.
03.Compare two isotopes
Higher
Question
Carbon-12 and Carbon-14 are both isotopes of carbon. State what is the same and what is different about their nuclei.
Solution
1. 1
  Same.
  $\text{Same number of protons (6 in each).}$
2. 2
  Different.
  $\text{Different number of neutrons: 6 in C-12, 8 in C-14.}$
3. 3
  Implications.
  $\text{Same chemistry, different mass, different nuclear stability (C-14 is radioactive).}$
Answer
Both have 6 protons. C-12 has 6 neutrons; C-14 has 8 neutrons. Same chemistry but different mass; C-14 is radioactive.
04.Ion formation
Foundation
Question
A neutral oxygen atom (Z = 8, A = 16) gains 2 electrons. Write its symbol with charge and state the number of protons, neutrons and electrons.
Solution
1. 1
  Charge from gained electrons.
  $\text{Gained 2 e}^- \Rightarrow \text{charge} = -2;\quad \text{Symbol: O}^{2-}$
2. 2
  Subatomic counts.
  $\text{Protons} = 8;\quad \text{Neutrons} = 16 - 8 = 8;\quad \text{Electrons} = 8 + 2 = 10$
Answer
O²⁻: 8 protons, 8 neutrons, 10 electrons.
05.Interpret alpha scattering
Higher
Question
Explain how the observations from the Rutherford alpha-scattering experiment led to the nuclear model of the atom.
Solution
1. 1
  Observation 1.
  $\text{Most α particles passed straight through → atom mostly empty space.}$
2. 2
  Observation 2.
  $\text{Some α deflected at small angles → met a region of positive charge.}$
3. 3
  Observation 3.
  $\text{A few bounced straight back → hit a tiny, dense, positive nucleus.}$
4. 4
  Model implications.
  $\text{This is incompatible with plum pudding. The nuclear model replaced it: nucleus is small, dense and positive; electrons orbit at distance.}$
Answer
Most went through (empty space); small deflections (positive charge); rare bounce-back (small dense nucleus). Together → nuclear model with concentrated positive nucleus and mostly empty space.

Practice with flashcards

Card 1 of 110 known · 0 to review

Key definitions and keywords

NucleusExaminer keyword: The dense central region of an atom containing protons and neutrons.
ProtonExaminer keyword: Positively charged subatomic particle in the nucleus. Relative charge +1, relative mass 1.
NeutronExaminer keyword: Neutral subatomic particle in the nucleus. Relative mass 1, no charge.
ElectronExaminer keyword: Negatively charged particle orbiting the nucleus in specific energy levels. Relative charge −1, relative mass ~1/2000.
Atomic number (Z)Examiner keyword: The number of protons in the nucleus of an atom. Defines the element.
Mass number (A)Examiner keyword: The total number of protons and neutrons in the nucleus.
IsotopeExaminer keyword: An atom of the same element (same Z) with a different number of neutrons (different A).
IonExaminer keyword: A charged atom (or molecule), where the number of electrons differs from the number of protons.
Plum-pudding modelExaminer keyword: Thomson's 1897 model: an atom is a sphere of positive 'dough' with electrons embedded like plums.
Nuclear modelExaminer keyword: Rutherford's model: a small, dense, positively-charged nucleus surrounded by mostly empty space in which electrons orbit.
Bohr modelExaminer keyword: Electrons orbit in fixed energy levels (shells); explains electron stability and emission spectra.

Common mistakes and misconceptions

Mistake
Saying nucleus and atom are the same size.
Why it happens
Pictures aren't to scale.
How to avoid it
Atom diameter 10⁻¹⁰ m vs nucleus 10⁻¹⁵ m — nucleus is 100,000× smaller.
Mistake
Treating electron mass as significant.
Why it happens
Each subatomic particle 'counts'.
How to avoid it
Electron mass is ~1/2000 of a nucleon — negligible at GCSE.
Mistake
Confusing Z and A.
Why it happens
Both are numbers on the symbol.
How to avoid it
Z (small number, bottom-left) = protons; A (larger, top-left) = protons + neutrons.
Mistake
Saying isotopes are different elements.
Why it happens
They have different mass numbers.
How to avoid it
Same Z = same element. They are different ATOMS of the same element.
Mistake
Thinking ions have a different number of protons.
Why it happens
Ions feel 'changed'.
How to avoid it
Only electrons change. Protons stay the same (else it would be a different element).
Mistake
Mixing up scientists and contributions.
Why it happens
Many scientists involved.
How to avoid it
Memorise: Dalton (spheres), Thomson (electron, plum pudding), Rutherford (nucleus), Bohr (energy levels), Chadwick (neutron).
Mistake
Saying 'most particles bounced back'.
Why it happens
Confusing the surprising minority with the bulk result.
How to avoid it
Most passed STRAIGHT THROUGH. A tiny fraction bounced back — the surprise that revealed the nucleus.

Atoms and Isotopes

Master the topic

Step-by-step worked examples

01.Comparing nucleus and atom size

02.Read nuclear notation

03.Compare two isotopes

04.Ion formation

05.Interpret alpha scattering

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions

Atoms and Isotopes

Master the topic

Step-by-step worked examples

01.Comparing nucleus and atom size

02.Read nuclear notation

03.Compare two isotopes

04.Ion formation

05.Interpret alpha scattering

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions